Crystal Growth and Dissolution of Methylammonium Lead Iodide Perovskite in Sequential Deposition: Correlation between Morphology Evolution and Photovoltaic Performance

Performance Improvement of Lead-Based Halide Perovskites through B-Site Ion-Doping Strategies

Owing to their excellent properties, lead halide perovskites have attracted extensive attention in the photoelectric field. Presently, the certified power conversion efficiency of perovskite solar cells has reached 25.7%, the specific detectivity of perovskite photodetectors has exceeded 1014 Jones, and the external quantum efficiency of perovskite-based light-emitting diode has exceeded 26%. However, their practical applications are limited by the inherent instability induced by the perovskite structure due to moisture, heat, and light. Therefore, one of the widely used strategies to address the issue is to replace partial ions of the perovskites with ions of smaller radii to shorten the bond length between halides and metal cations, improving the bond energy and enhancing the perovskite stability. Particularly, the B-site cation in the perovskite structure can affect the size of eight cubic octahedrons and their gap. However, the X-site can only affect four such voids. This review comprehensively summarizes the recent progress in B-site ion-doping strategies for lead halide perovskites and provides some perspectives for further performance improvements.

Perovskite single-crystal thin films: preparation, surface engineering, and application

Perovskite single-crystal thin films (SCTFs) have emerged as a significant research hotspot in the field of optoelectronic devices owing to their low defect state density, long carrier diffusion length, and high environmental stability. However, the large-area and high-throughput preparation of perovskite SCTFs is limited by significant challenges in terms of reducing surface defects and manufacturing high-performance devices. This review focuses on the advances in the development of perovskite SCTFs with a large area, controlled thickness, and high quality. First, we provide an in-depth analysis of the mechanism and key factors that affect the nucleation and crystallization process and then classify the methods of preparing perovskite SCTFs. Second, the research progress on surface engineering for perovskite SCTFs is introduced. Third, we summarize the applications of perovskite SCTFs in photovoltaics, photodetectors, light-emitting devices, artificial synapse and field-effect transistor. Finally, the development opportunities and challenges in commercializing perovskite SCTFs are discussed.

Manipulation of Crystallization Kinetics for Perovskite Photovoltaics Prepared Using Two-Step Method

Two-step fabricated perovskite solar cells have attracted considerable attention because of their good reproducibility and controllable crystallization during production. Optimizing the quality of perovskite films plays a decisive role in realizing superb performance via a two-step method. Many breakthroughs have been achieved to obtain high-quality film from the perspective of manipulating crystallization kinetics in the two-step preparation process, which promotes the rapid development of perovskite photovoltaics. Therefore, focusing on the crystallization process in the two-step preparation process can provide a reliable basis for optimizing the performance of two-step devices. In this review, recent progress on regulating the crystallization process for two-step PSCs is systematically reviewed. Firstly, a specific description and discussion are provided on the crystallization process of perovskite in different two-step methods, including spin-coating, immersion and evaporation. Next, to obtain high-quality perovskite film via these two-step methods, current strategies of additive engineering, composition engineering, and solvent engineering for regulating the crystallization process for two-step perovskite are classified and investigated. Lastly, the challenges which hindering the performance of the two-step perovskite photovoltaics and an outlook toward further developments are proposed.

Importance of methylammonium iodide partial pressure and evaporation onset for the growth of co-evaporated methylammonium lead iodide absorbers

Vacuum-based co-evaporation promises to bring perovskite solar cells to larger scales, but details of the film formation from the physical vapor phase are still underexplored. In this work, we investigate the growth of methylammonium lead iodide (MAPbI[Formula: see text]) absorbers prepared by co-evaporation of methylammonium iodide (MAI) and lead iodide (PbI[Formula: see text]) using an in situ X-ray diffraction setup. This setup allows us to characterize crystallization and phase evolution of the growing thin film. The total chamber pressure strongly increases during MAI evaporation. We therefore assume the total chamber pressure to be mainly built up by an MAI atmosphere during deposition and use it to control the MAI evaporation. At first, we optimize the MAI to PbI[Formula: see text] impingement ratios by varying the MAI pressure at a constant PbI[Formula: see text] flux rate. We find a strong dependence of the solar cell device performance on the chamber pressure achieving efficiencies > 14[Formula: see text] in a simple n-i-p structure. On the road to further optimizing the processing conditions we vary the onset time of the PbI[Formula: see text] and MAI deposition by delaying the start of the MAI evaporation by t = 0/8/16 min. This way, PbI[Formula: see text] nucleates as a seed layer with a thickness of up to approximately 20 nm during this initial stage. Device performance benefits from these PbI[Formula: see text] seed layers, which also induce strong preferential thin film orientation as evidenced by grazing incidence wide angle X-ray scattering (GIWAXS) measurements. Our insights into the growth of MAPbI[Formula: see text] thin films from the physical vapor phase help to understand the film formation mechanisms and contribute to the further development of MAPbI[Formula: see text] and related perovskite absorbers.

Methylammonium Polyiodides in Perovskite Photovoltaics: From Fundamentals to Applications

Discovered in 2017, methylammonium polyiodides were proposed as a facile precursor for synthesis of hybrid perovskites by means of their interaction with metallic lead, which initiated further active exploration of their potential applications. Investigation of their unusual properties such as liquid state, unprecedented phase diversity and high reactivity revealed that methylammonium polyiodides are the first representatives of a new class of compounds-reactive polyhalide melts (RPM). In this review, we summarize the reported data on the unique properties of these compounds, discuss their potential for fabrication of hybrid perovskite films and describe the role of polyhalides in degradation of perovskite solar cells.

New Acidic Precursor and Acetone-Based Solvent for Fast Perovskite Processing via Proton-Exchange Reaction with Methylamine

A new solvent system for PbI₂ based on HI solution in acetone with a low boiling point is proposed. High solubility of PbI₂ is caused by the formation of iodoplumbate complexes, and reaches a concentration of 1.6 M. Upon its crystallization metastable solvate phases PbI₂∙HI∙n{(CH₃)₂CO} are formed. The latter allows for their easy deposition on substrates in a form of smooth and uniform thin films by spin-coating. Through a fast acid-base reaction with a gaseous amine, the films of the intermediate phase can be completely converted to single-phase perovskite films. The developed method allows one to form smooth perovskite films with high crystallinity with a thickness up to 1 μm. Due to easy and fast processing, the developed method can be promising for perovskite technology upscaling.

Tuning Magnetism and Photocurrent in Mn-Doped Organic-Inorganic Perovskites

Organic-inorganic perovskites have attracted increasing attention in recent years owing to their excellent optoelectronic properties and photovoltaic performance. In this work, the prototypical hybrid perovskite CH₃NH₃PbI₃ is turned into a ferromagnetic material by doping Mn, which enables simultaneous control of both charge and spin of electrons. The room-temperature ferromagnetism originates from the double exchange interaction between Mn²⁺-I^--Mn³⁺ ions. Furthermore, it is discovered that the magnetic field can effectively modulate the photovoltaic properties of Mn-doped perovskite films. The photocurrent of Mn-doped perovskite solar cells increases by 0.5% under a magnetic field of 1 T, whereas the photocurrent of undoped perovskite decreases by 3.3%. These findings underscore the potential of Mn-doped perovskites as novel solution-processed ferromagnetic material and promote their application in multifunctional photoelectric-magnetic devices.

Europium and Acetate Co-doping Strategy for Developing Stable and Efficient CsPbI2 Br Perovskite Solar Cells

All-inorganic perovskite solar cells have developed rapidly in the last two years due to their excellent thermal and light stability. However, low efficiency and moisture instability limit their future commercial application. The mixed-halide inorganic CsPbI₂ Br perovskite with a suitable bandgap offers a good balance between phase stability and light harvesting. However, high defect density and low carrier lifetime in CsPbI₂ Br perovskites limit the open-circuit voltage (V_oc < 1.2 V), short-circuit current density (J_sc < 15 mA cm^-2 ), and fill factor (FF < 75%) of CsPbI₂ Br perovskite solar cells, resulting in an efficiency below 14%. For the first time, a CsPbI₂ Br perovskite is doped by Eu(Ac)₃ to obtain a high-quality inorganic perovskite film with a low defect density and long carrier lifetime. A high efficiency of 15.25% (average efficiency of 14.88%), a respectable V_oc of 1.25 V, a reasonable J_sc of 15.44 mA cm^-2 , and a high FF of 79.00% are realized for CsPbI₂ Br solar cells. Moreover, the CsPbI₂ Br solar cells with Eu(Ac)₃ doping demonstrate excellent air stability and maintain more than 80% of their initial power conversion efficiency (PCE) values after aging in air (relative humidity: 35-40%) for 30 days.

Doped Manipulation of Photoluminescence and Carrier Lifetime from CH3NH3PbI3 Perovskite Thin Films

The compositional doping techniques can delicately tune the band gap, carrier concentration, and mobility of perovskites to optimize the photoelectric properties of materials. It is reported that the doped perovskites have been widely researched in the photovoltaic and photoelectronic field. Here, we show that the photoluminescence intensity and carrier lifetime of CH₃NH₃PbI₃ films have been improved by 3 orders of magnitude by incorporating abundant MnAc₂·4H₂O in the perovskite precursor solution, which benefits from the morphological change and surface passivation induced by hydration water and surface manganese acetate. We also witness the increased photoluminescence quantum yield for film and the changed power conversion efficiency for perovskite solar cells. More importantly, the enhanced chemical stability of perovskite is displayed by immersing films into the water.

Distinguishing crystallization stages and their influence on quantum efficiency during perovskite solar cell formation in real-time

Relating crystallization of the absorber layer in a perovskite solar cell (PSC) to the device performance is a key challenge for the process development and in-depth understanding of these types of high efficient solar cells. A novel approach that enables real-time photo-physical and electrical characterization using a graphite-based PSC is introduced in this work. In our graphite-based PSC, the device architecture of porous monolithic contact layers creates the possibility to perform photovoltaic measurements while the perovskite crystallizes within this scaffold. The kinetics of crystallization in a solution based 2-step formation process has been analyzed by real-time measurement of the external photon to electron quantum efficiency as well as the photoluminescence emission spectra of the solar cell. With this method it was in particular possible to identify a previously overlooked crystallization stage during the formation of the perovskite absorber layer. This stage has significant influence on the development of the photocurrent, which is attributed to the formation of electrical pathways between the electron and hole contact, enabling efficient charge carrier extraction. We observe that in contrast to previously suggested models, the perovskite layer formation is indeed not complete with the end of crystal growth.

DMF as an Additive in a Two-Step Spin-Coating Method for 20% Conversion Efficiency in Perovskite Solar Cells

DMF as an additive has been employed in FAI/MAI/IPA (FA= CH₂(NH₂)₂, MA = CH₃NH₃, IPA = isopropanol) solution for a two-step multicycle spin-coating method in order to prepare high-quality FA_xMA_1-xPbI_2.55Br_0.45 perovskite films. Further investigation reveals that the existence of DMF in the FAI/MAI/IPA solution can facilitate perovskite conversion, improve the film morphology, and reduce crystal defects, thus enhancing charge-transfer efficiency. By optimization of the DMF amount and spin-coating cycles, compact, pinhole-free perovskite films are obtained. The nucleation mechanisms of perovskite films in our multicycle spin-coating process are suggested; that is, the introduction of DMF in the spin-coating FAI/MAI/IPA solution can lead to the formation of an amorphous phase PbX₂-AI-DMSO-DMF (X = I, Br; A = FA, MA) instead of intermediate phase (MA)₂Pb₃I₈·2DMSO. This amorphous phase, similar to that in the one-step method, can help FAI/MAI penetrate into the PbI₂ framework to completely convert into the perovskite. As high as 20.1% power conversion efficiency (PCE) has been achieved with a steady-state PCE of 19.1%. Our work offers a simple repeatable method to prepare high-quality perovskite films for high-performance PSCs and also help further understand the perovskite-crystallization process.